Is it dangerous to top up zinc-chloride AA cells?

Yep

With a correctly-designed charger, no. Sticking them across a car battery, yes.

-- Sue

I did it many decades ago as a child. You can't put much back into them, and after a few cycles, the zinc case will likely give up, so you might want to use leakproof ones. Also, you have to charge very slowly to prevent gas being given off.

They're likely to leak and might wreck some appliance.